PostHog

PostHog is an open‑source, self‑hosted product analytics platform that bundles event collection, real‑time session replay, feature flagging, experiments, error tracking, and data warehousing into a single stack. From a developer’s perspective it behaves like a modular observability system: you instrument your front‑end or back‑end with tiny SDKs, and the platform ingests, stores, and surfaces that data via a GraphQL/REST API, SQL editor, and ready‑made dashboards. The core goal is to give product teams the same breadth of insight as a SaaS offering while keeping full control over data residency and cost.

Technical Stack & Architecture

PostHog’s core is written in Python (FastAPI) and TypeScript/React for the UI. The event ingestion layer uses a high‑throughput Kafka cluster (or its lightweight “event bus” alternative) to buffer events before persisting them in ClickHouse, a columnar database optimized for analytical queries. Feature flags and experiments are stored in PostgreSQL, while session replay blobs live in S3‑compatible object storage. The architecture is deliberately microservice‑oriented: separate services for ingestion, analytics, CDP pipelines, and the web UI can be scaled independently. Docker Compose and Helm charts make it trivial to spin up a single‑node dev environment or a multi‑zone production cluster.

Core Capabilities & APIs

• Event Capture – Autocapture SDKs for web, iOS/Android, and server‑side libraries expose a posthog.capture(eventName, properties) API. Events are enriched with user context and forwarded to the ingestion pipeline.
• SQL & GraphQL – A built‑in SQL editor against ClickHouse allows ad‑hoc analysis, while a GraphQL endpoint (/api/graphql) provides typed access to events, cohorts, and feature flag status.
• Feature Flags & Experiments – A REST API (/api/feature_flags) lets developers toggle flags programmatically; the experiment service exposes an A/B test API that returns bucket assignments and logs conversion events.
• Session Replay – The replay SDK streams user interactions to a WebSocket endpoint; the UI stitches them into a timeline that can be queried via GET /api/replays/{id}.
• Webhooks & Destinations – The CDP layer supports 25+ outbound connectors and a generic webhook endpoint (/api/webhooks) that can be triggered on any event or transformation.

Deployment & Infrastructure

PostHog ships as a Docker image, with official Helm charts for Kubernetes. A single‑node installation requires 4 GB RAM and a ClickHouse instance; production clusters typically deploy three replicas of each service for high availability. The platform supports auto‑scaling via Kubernetes HPA and can ingest millions of events per day with minimal latency. Data retention policies are configurable at the database level, and backups can be scheduled to object storage or external databases.

Integration & Extensibility

The plugin system is exposed through a simple Python API (posthog.plugins). Developers can write custom event processors, transform data before it hits ClickHouse, or expose new dashboards. The CDP pipeline is fully programmable in Python and supports custom SQL transformations. Additionally, PostHog’s SDKs can be extended with middleware to inject authentication tokens or enrich events with telemetry from third‑party services.

Developer Experience

Documentation is organized by feature set and includes code samples for every SDK. The community is active on GitHub, Discord, and the PostHog forum; PRs are merged rapidly (PR‑friendly). The open source license (MIT) removes vendor lock‑in, and the platform’s modularity means you can drop in only the services you need. The built‑in CI pipeline for plugins and SDKs further lowers the barrier to contribution.

Use Cases

• Product‑Led Growth: Capture funnel analytics and run experiments without leaving the product stack.
• Compliance‑Heavy Domains: Host all user telemetry on premises to satisfy GDPR or HIPAA requirements.
• Full‑Stack Observability: Combine error tracking, session replay, and feature flag data into a single queryable store.
• Data‑Warehouse Integration: Sync ClickHouse events to Snowflake or BigQuery for downstream BI tools.

Advantages Over Alternatives

PostHog delivers the breadth of a SaaS analytics platform with the performance and flexibility of an on‑prem solution. Its columnar ClickHouse backend gives sub‑second query times at scale, while the modular architecture lets you spin up only what you need. The open source license eliminates recurring costs, and the robust SDK ecosystem ensures rapid iteration. For developers who need full control over data flow, low‑latency analytics, and the ability to extend every layer of the stack, PostHog is a compelling choice.